U.S. patent number 5,533,873 [Application Number 08/508,891] was granted by the patent office on 1996-07-09 for induction regulator valve for rotary compressors.
This patent grant is currently assigned to Hoerbiger Ventilwerke Aktiengesellschaft. Invention is credited to Gerhard Kindl.
United States Patent |
5,533,873 |
Kindl |
July 9, 1996 |
**Please see images for:
( Certificate of Correction ) ** |
Induction regulator valve for rotary compressors
Abstract
An induction regulator valve is incorporated in the induction
manifold of a compressor to provide for infinitely variable
regulation of the output from the rotary type compressor, in
particular the screw-type compressor. The closing piece of the
induction regulator valve can be adjusted by means of an actuator
system. To improve this regulation performance of this system, in
particular to reduce its sensitivity when the regulator valve is
almost closed, the spring system is formed so that its stiffness
varies along the length of its spring path, the stiffness of the
spring system being smaller when the induction regulator valve is
open than when it approaches the closed position of the closing
piece of the induction regulator valve.
Inventors: |
Kindl; Gerhard (Vienna,
AU) |
Assignee: |
Hoerbiger Ventilwerke
Aktiengesellschaft (Vienna, AT)
|
Family
ID: |
3515224 |
Appl.
No.: |
08/508,891 |
Filed: |
July 28, 1995 |
Foreign Application Priority Data
|
|
|
|
|
Jul 29, 1994 [AU] |
|
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A-1506.94 |
|
Current U.S.
Class: |
417/295; 137/529;
417/298; 418/201.2 |
Current CPC
Class: |
F04B
49/225 (20130101); F04C 28/24 (20130101); F16K
31/1221 (20130101); F04C 18/16 (20130101); F04B
2205/05 (20130101); F04C 29/026 (20130101); F04C
2270/185 (20130101); Y10T 137/7905 (20150401) |
Current International
Class: |
F04B
49/22 (20060101); F16K 31/122 (20060101); F04B
049/00 () |
Field of
Search: |
;417/295,298,441
;137/529 ;418/201.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Freay; Charles
Attorney, Agent or Firm: Watson Cole Stevens Davis
Claims
The Embodients of the Invention in Which an Exclusive Property or
Privilege is Claimed are Defined as Follows:
1. An induction regulator valve for installation in the induction
manifold of a rotary compressor and operable by a pneumatic
actuator, comprising:
a closure member for regulating fluid flow through the valve;
a displaceable adjusting member accommodating sad closure member
and responsive to a pressurized medium to determine an operating
position of the closure member that can lie between a closed
position and an open position; and
spring means operable to oppose the action of said pressurized
medium on an adjusting member, said spring means having a varying
stiffness that is less when the operating position of said closure
member is closer to its open position than when said closure member
is closer to its closed position.
2. An induction regulator valve as claimed in claim 1, wherein said
spring means comprises at least one coil spring, the pitch of the
turns of said spring varying along the length of the spring.
3. An induction regulator valve as claimed in claim 1, wherein said
spring means comprises at least one coil spring, the turns of said
spring being of different cross-section.
4. An induction regulator valve as claimed in claim 1, wherein said
spring means comprises a disk spring having a progressive spring
rate.
5. An induction regulator valve as claimed in claim 1, wherein said
spring means comprises at least one spring having a progressing
spring rate, said spring being made of an elastomer.
6. An induction regulator valve as claimed in claim 1, wherein said
spring means comprises a coil spring, a part of the spring turns of
which being located within a helical channel of a support for
rendering said part of the spring turns ineffective when the spring
is compressed.
7. An induction regulator valve as claimed in claim 1, wherein the
spring system comprises at least two springs of different
stiffness, and which are arranged coaxially one above the other and
held together by friction.
8. An induction regulator valve as claimed in claim 7, wherein said
at least two springs are held together by friction through an
interposed spring collar.
9. An induction regulator valve as claimed in claim 1, wherein said
spring means comprises at least two springs of different stiffness
that are arranged coaxially one above the other and held together
by friction by means of at least one disk spring, an end stop being
provided on a housing of the pneumatic actuator for at least one
said spring disk.
10. An induction regulator valve as claimed in claim 1, wherein the
spring means comprises at least two springs of different diameter,
said springs being arranged coaxially one inside the other and
being of different lengths.
11. An induction regulator valve as claimed in claim 10, wherein
said springs are of different stiffness.
12. An induction regulator valve as claimed in claim 1, wherein
said spring means comprises at least two springs of different
diameter, said springs being arranged concentrically inside each
other, and a stop is provided for at least one of said springs,
said stop initially supporting a free end of said one spring.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an induction regulator valve for
rotary compressors, in particular for screw-type compressors.
The valve is installed in the induction manifold of the compressor
and can be operated by a pneumatic actuator, the adjusting body of
which, for example, pistons or diaphragms, that is acted upon by a
pressurized medium, adjusts the closing piece of the induction
regulator valve against a spring system that acts on the closing
piece of the induction regulating valve or on the adjusting body of
the actuator system.
It is known that the delivery capacity of compressors, particularly
of rotary compressors, such as screw-type compressors or the like,
can be regulated by changing the throughput cross section of the
induction manifold, that is to say that the quantity of medium that
is brought in is throttled to a greater or lesser degree. This
throttling effect is achieved with the help of an induction
regulator valve of the type described in the introduction hereto.
This is incorporated into the induction manifold of the rotary
compressor, held in the open position by a spring system, and
adjusted in the closing direction by means of the actuator, which
as a rule is acted upon by compressed medium from the regulated
compressor itself.
Using this relatively simple arrangement, it is possible to
regulate the output from a rotary compressor across the whole
delivery range, so that for all practical purposes it is infinitely
variable, and can be matched to a particular requirement. In
practice, however, it has been shown that only relatively precise
regulation is possible using this known arrangement and, in
particular in the event of pronounced throttling of the induction
manifold, regulation becomes relatively unstable, particularly if
the cross section of the induction regulator valve is so selected
that in the wide-open position, there is a small valve loss.
It has been shown that, starting from an open induction regulator
valve, this can be closed to more than 80 percent of its stroke
without initiating a throttling effect of anything like a
corresponding order of magnitude. Only relatively small adjusting
forces are required in this range, and regulation is relatively
insensitive and stable. However, as the induction regulator valve
approaches the closed position, even a relatively small change in
its stroke will induce a relatively large change in the throttling
effect. Thus, regulation becomes ever more sensitive and unstable
as the closed position of the induction regulator is approached.
Even small changes in the adjusting pressure acting on the actuator
lead to a relatively large change in the throttling effect.
It is an object of the present invention to improve infinitely
variable regulation using an induction regulator valve. In
particular, it is intended to provide more stable regulation for
the same regulation variation or smaller variations of regulation
with adequate stability and to provide more precise regulation
overall.
SUMMARY OF THE INVENTION
According to the present invention the spring system provides for
varying degrees of stiffness over its spring travel, the stiffness
of the spring being less when the induction regulator valve is open
than it is when the closing piece of the induction regulator valve
is nearly in the closed position.
Surprising results are achieved by modifying or supplementing the
spring system in this manner. Starting with an open induction
regulator valve, the spring stiffness is slight. Accordingly, only
a small change of the adjusting pressure is required to achieve a
relatively large adjustment of the closing piece in the range in
which regulation is insensitive. This results from the type of
valve and is also a consequence of the dimensioning of the valve
cross section for a small valve loss when the valve is fully open.
In contrast to this, in the vicinity of the closed position of the
induction regulator valve, spring stiffness is considerably
greater, with the result that in this range the sensitivity of the
regulation is reduced and as a consequence it is correspondingly
more stable. Because of the relatively great spring stiffness, a
correspondingly large change in adjusting pressure is required in
order to achieve a relatively small adjustment of the closing piece
of the induction regulator valve and thus of the throttling effect
that is achieved. For this reason, regulation with the induction
regulator valve according to the present invention is distinguished
by great stability across the whole range of regulation, so that
better regulation is achieved than is the case with conventional
induction regulator valves.
The present invention can be used both for seat-type valves that
are used as induction regulator valves and for butterfly valves or
slides. The present invention is independent of the type of
induction regulator valve itself. All that is important is that the
closing piece of the induction regulator valve be adjusted against
the force of a spring system and the stiffness of this spring
system be considerably greater in the vicinity of the closed
position of the induction regulator than it is in the vicinity of
the open position of said valve.
A number of different embodiments of the spring system according to
the present invention are possible within the context of the
invention. The spring system can consist of at least one coil
spring in which the pitch of the spring coils differs along the
length of the spring. When the spring is compressed, essentially it
is first the spring coils with the least pitch that are effective
and on further compression of the spring these lie against each
other. The stiffness of the spring becomes similarly greater
because of the greater pitch of the remaining coils.
In another embodiment of the present invention, the spring system
consists of at least one coil spring, the coils of which are of
different diameter or cross section. In this case, the first to
become effective are the coils with the greatest spring diameter
and/or the smallest cross-section because the stiffness of these
coils is relatively slight. Greater stiffness is achieved with
small coil diameters and/or a greater cross section of the wire
forming the spring.
The spring system can also consist of a disk or plate-type spring
that has a progressive spring rate. It is also possible to use a
disk or plate-type spring that does not have a progressive spring
rate, when this is then supported on a spring collar having at
least one stop that projects against it. On compression, the disk
spring lies progressively against the stop from the outside toward
the inside. Whereupon the effective spring length is reduced and
the stiffness of the spring is thus increased progressively.
In another embodiment, the spring system consists of at least one
spring that is manufactured from an elastomer, e.g., rubber, this
spring having a progressive spring rate.
A progressive spring rate for the spring can also be achieved if
the spring system consists of a coil spring that has a section of
its length inserted into a helical channel in a supporting element,
which bridges some of the coils of the spring when the spring is
compressed. The coils of the spring that are arranged within the
channel of the supporting element lie against the supporting
element and are non-effective, which means that the stiffness of
the spring as a whole is increased.
In an advantageous embodiment of the present invention, the spring
system consists of at least two springs of different stiffness.
These are arranged coaxially above one another and are connected to
each other by friction, for example, through an interposed spring
collar. Also in this embodiment, starting with the regulator valve
open, it is initially the spring with the least stiffness that
becomes effective, whereas the spring having greater stiffness
first becomes effective in the vicinity of the closed position of
the induction regulator valve, or when this induction regulator
valve is almost closed.
Another embodiment of the spring system consists of at least two
springs of different stiffness that are arranged coaxially one
above the other and joined together by friction through a spring
collar such that an end stop for at least one spring collar is
provided on the housing of the actuator. The particular spring that
is supported on this spring collar then remains effective until
such time as the spring collar comes to rest against its end stop.
Then, a spring that is of greater stiffness becomes effective.
In a preferred embodiment of the present invention, provision is
made such that the spring system consists of at least two springs
of different diameter, these springs, which are of different
lengths, being arranged coaxially one above the other and joined
together by friction through a spring collar. The spring with the
least stiffness is best configured so as to be longer, in contrast
to which the spring that is of greater stiffness is relatively
short. When the springs are compressed, which is to say as the
induction regulator valve closes progressively, initially it is
only the spring with the lesser stiffness that is effective. Only
in the vicinity of the closed position of the regulator valve does
the shorter and stiffer spring come into contact with the housing
of the actuator, so that on further adjustment both springs are
compressed and the spring stiffness of the spring system becomes
correspondingly greater.
The springs of this spring system that are arranged coaxially one
inside another can be of identical stiffness, when the different
stiffness of the spring system itself, taken as a whole, is
achieved in that a different member of springs becomes effective.
The springs of the spring system can, however, be of different
stiffness. The spring system according to the present invention can
thus be manufactured with any desired characteristics in a very
simple manner.
Finally, another embodiment of the present invention is such that
the springing consists of at least two springs of different
diameter, these being arranged concentrically one inside the other,
and in that for at least one of these springs there is a stop that
supports the end of the spring that is initially unattached. Also,
in this embodiment, the stiffness of the spring system as a whole
becomes correspondingly greater when the unattached end of the
spring comes into contact with the stop that is provided.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail, by way of
example only, with reference to the accompanying drawings, in
which:
FIGS. 1 and 2 are diagrams showing a compressor system
incorporating an induction regulator valve according to the present
invention;
FIG. 3 is an axial cross-sectional view taken through one
embodiment of the regulator valve with the actuator;
FIG. 4 is an axial mid-line sectional view taken through another
embodiment of the actuator without the valve; and
FIGS. 5 to 16 are axial line sectional view taken through different
embodiments of the actuator, these being shown in some cases with
the valve omitted.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The compressor systems that are shown in FIG. 1 and FIG. 2 consist
in each instance of a screw-type compressor 1 that is driven by a
motor 2. In the embodiment that is shown in FIG. 1 this is an
electric motor, and in the embodiment that is shown in FIG. 2, it
is a diesel engine. In each instance, the motor 2 is indicated only
diagrammatically; the power supply to the electric motor and the
fuel system and all auxilliary systems for the diesel engine having
been omitted.
In both embodiments, the compressor has an induction manifold 3
with an incorporated filter 4. In addition, an induction regulator
valve 5, shown diagrammatically in FIGS. 1 and 2 is incorporated in
the induction manifold. A pressure line 6 leads from the screw-type
compressors 1 to a pressure accumulator 7 formed as an oil
separator, and a fine separator 8 is arranged in the lower part
thereof. A supply line 10 leads therefrom through a minimum
pressure valve 9 to a consumer, for example a compressed air
system, not shown.
The minimum pressure valve 9 shown has a non-return valve that
closes when the pressure is released from the pressure accumulator
7, thereby preventing simultaneous release of pressure at the
consumer. The minimum pressure valve 9 also closes if the pressure
at the consumer falls below a minimum value so as to prevent too
rapid a pressure drop in the pressure accumulator 7, which would
endanger the lubrication and cooling of the compressor 1. Thus, the
minimum pressure valve 9 ensures that the pressurized medium is
always at adequate pressure, so as to ensure that liquid is
supplied from the pressure accumulator 7 to the screw-type
compressor 1. To this end, a fluid line 12 leads from the lower
part of the pressure accumulator 7 to the screw-type compressor 1.
The fluid that is delivered through the lines 12, 13 serves to
seal, cool, and lubricate the screw-type compressor. A filter 14
and a cooler 15 are incorporated in the fluid line 12. The
relatively small quantity of fluid that collects in the fine
separator 8 is returned to screw-type compressor 1 via a further
fluid line 13.
The arrangements according to FIGS. 1 and 2 also include a control
line 16 leading from the pressure accumulator 7 and branching to
power a solenoid valve 17 and a proportional regulator 18. The
solenoid valve 17 is configured as an idling- and ventilating
valve. The proportional regulator 18 is adjustable and regulates
the pressure within the pressure accumulator 7. As the compressor
pressure rises, the proportional regulator 18 opens and the
pressure in the control line that leads to the induction regulator
valve 5 rises. When the compressor pressure falls, the proportional
regulator 18 closes and the pressure in the control line 20 is
reduced by being blown off through an exhaust nozzle 19. The
pressurized medium that is supplied to the induction regulator
valve 5 through a further control line 20 operates the actuator of
the induction regulator valve 5 and thus regulates the quantity
supplied to the screw-type compressors 1 according to the pressure
that had been set at the proportional regulator 18.
In the embodiment that is shown in FIG. 2, the induction regulator
valve 5 is in the form of a butterfly-type control valve that is
adjusted by means of a lever-type control 21, and an actuator 22
that is similarly shown only in diagrammatic form is provided for
this purpose. A further actuator 23, supplied by way of a control
line 24, controls the injection pump (not shown) of the drive motor
2, which in this example is a diesel engine.
FIG. 3 shows an induction regulator valve 5 of the kind used in the
compressor system shown in FIG. 1. This valve comprises a housing
26 having a cover 28 secured thereto by the flange bolts 27 and
through which the suction line 28a passes; this simultaneously
forms the valve seat for the closing piece 29 of the induction
regulator valve 5.
This closing piece 29 is supported in the hollow adjusting body 30
of an actuator 31, against the force of a weak spring 32. The
adjusting body 30 is configured as a piston and is sealed and
guided within a cylinder 33. Pressurized medium can be supplied to
adjust the adjusting body 30 formed as a piston through a connector
bore, 34 from which a channel 35 leads into the cylinder 33. This
adjusting body 30 is acted upon by a spring system 11, comprising
two return springs 36 and 37 that are arranged concentrically
around it.
The two return springs 36 and 37 that make up the spring system 11
of the actuator 31 are of almost equal cross sectional area and
both have nearly identical pitch. The outer spring 36 is not as
stiff as the inner spring 37. In addition, the return spring 36
extends to the whole stroke of the actuator, in contrast to which
the inner return spring 37 is shorter.
FIG. 3 shows that the closing piece 29 holds the induction
regulator valve 5 closed in the position shown. This applies to any
position the induction regulator valve 5 is in when the compressor
1 is stationary. When this happens, the closing piece 29 held onto
its seat by the weak spring 32 prevents compressed medium from
flowing back from the pressure accumulator 7 into the suction line
28a when the compressor 1 is not operating. As soon as the
compressor 1 begins to run, however, the suction overcomes the
force of the weak spring 32, whereupon the adjusting body 29 opens
the induction regulator valve 5.
In order to close the induction regulator valve 5 either completely
or partially when the compressor is operating, and thereby throttle
the induction of medium in order to regulate the output, medium
that is under pressure is supplied through the connector 34 and the
channel 35. This acts upon the adjusting body formed as a piston,
and moves this against the force of the spring system 11, initially
upwards against the force of the return spring 36, when it then
moves the closing piece 29 in the direction of its seat. Since the
second return spring 37 is not initially under load, movement of
the adjusting body 30 requires only a relatively low medium
pressure. However, as soon as the second return spring 37 comes
into contact with the upper end of the cover 38 of the actuator 31,
it becomes effective, whereupon it increases the stiffness of the
spring system 11 correspondingly. For this reason, considerably
greater pressure of the pressurized medium is required to move the
closing piece 29 further in the vicinity of its closed position,
and this means that regulation is correspondingly more stable and
precise so that, overall, improved regulation performance is
achieved.
The actuator 22 used to adjust the regulator butterfly valve (not
shown) in the FIG. 2 embodiment is shown in FIG. 4. This has a
cylinder housing 39 with a rolled diaphragm 40 within it; this
moves the butterfly valve of the regulator valve 5 a rod 41 that
passes out at the upper end via the link 21. In this embodiment, a
pressurized-medium connector 34 is also provided to move the rolled
diaphragm 40.
The rolled diaphragm 40 lies on a spring collar 42 connected to the
rod 41 and supports the spring system 11. This spring system 11
consists of two return strings 36 and 37 that are arranged
coaxially above each other with a disk spring 44 therebetween. The
springs 36, 37 are of identical diameter. The lower return spring
37 is, however, wound from much thicker wire than the return spring
36 that is located above it, and is thus considerably stiffer. When
the rolled diaphragm 40 is acted upon by pressurized medium through
the connector 34 it first compresses the weaker return spring 36,
until the spring collar 44 comes into contact with a stop 46 that
is located above it, concentrically around the rod 41. Then, the
stiffer return spring 37 becomes effective, so that from this
moment on, exactly as in the embodiment shown in FIG. 3, control
becomes less sensitive but, at same time, more stable. This
arrangement is used to achieve more precise and improved
regulation.
FIGS. 5 to 16 show several embodiments of spring systems 11 that
can be used in the induction regulator valve according to the
present invention, these springs being of different stiffness.
Essentially, all of these embodiments correspond to the induction
regulator valve 5 shown in FIG. 3, although in the embodiments only
the actuator 31 with the spring system 11 is shown. Thus, in each
embodiment, the actuator 31 has a cylinder 33 in which the
adjusting body 30 is guided and sealed. The closing body 29, of
which only its cylindrical guide piece is shown, can move in a
hollow chamber of the adjusting body 30, against the force of a
weak spring 32. The channel 35 for the delivery of pressurized
medium to operate the adjusting body 13 opens out into the lower
end of the cylinder 33; at its upper end, the cylinder 33 is closed
off by a cover 38.
In the embodiment shown in FIG. 5, the spring system 11 consists of
a single coil spring 36, and the pitch of the turns of this coil
spring varies along the length of the spring 36. As the stroke of
the spring increases, the turns of the spring lie progressively
against each other, so that reduction of the spring turns makes the
spring progressively stiffer.
In the embodiment that is shown in FIG. 6, there is, once again,
only a single coil spring 36 as the spring system 11. Some of the
spring turns, which are of identical pitch, lie in a helical
channel 47 of a support 48 lying on an adjusting body 30 that can
be moved against the spring system 11. In this embodiment, when the
spring stroke increases, the turns that are located in the channel
47 of the support 48 lie sequentially on the support 48 and are
thus rendered ineffective. When this happens, there is also an
increase of the spring stiffness because of the reduction of
effective turns.
The FIG. 7 embodiment includes a single coil spring 36, the coils
of which being of different cross-section. Thus, the first to
become effective are the coils having the greatest spring diameter
and/or the smallest cross-section given that the stiffness of such
coils is relatively slight. Greater stiffness is effected with
small coil diameters and/or a greater cross-section of the wire
forming the spring.
The spring system of FIG. 8 has a spring 36 comprised of an
elastomeric material, such as rubber, to effect a progressive
spring rate.
The spring 11 system of FIG. 9 comprises a conical disc spring 36
with a progressive spring rate.
In FIG. 10, spring 11 comprises a conical disc spring 36 seated
against a spring collar 38 having a disk shaped seating surface 49.
Thus, with increasing spring lift, the cup spring seats
progressively on surface 49, whereby the spring length is reduced
and the spring rigidity is increased.
FIGS. 11 to 16 show embodiments in which a spring system 11
consisting of more than one spring is provided. In FIG. 11 a coil
spring 36 is supported on the adjuster body 30 with a coaxial disk
spring 37 of greater stiffness located thereabove. Between the two
springs there is a spring collar 44. When this spring system 11 is
compressed, the turns of the coil spring 37, which is not as stiff,
first lie against each other. Then, the stiffer turns of the disk
spring 27 become effective.
In the embodiment shown in FIG. 12, a coil spring 36 of lesser
stiffness and a disk spring 37 of greater stiffness are arranged
coaxially one above the other, with a spring collar 44 interposed
therebetween. In addition, on the cover 38 of the actuator 31 there
is a stop 46 for the spring collar 44. When this spring system 11
is placed under load, initially the coil spring 36 is compressed
until the spring collar 44 comes into contact with the stop 46.
When this happens, the stiffer disk spring 37 becomes effective,
and this results in progressive springing.
For the most part, the embodiments that are shown in FIGS. 13 and
14 compare with those shown in FIGS. 11 and 12. The difference lies
in the fact that instead of an elastomeric disk spring, a spring 37
having a progressive spring rate is used. In FIG. 13, the
elastomeric spring 37 is coaxial with the coil spring 36, with the
spring collar 44 interposed therebetween. FIG. 14 shows an
identical arrangement, except that the cover 38 has a stop 46 for
the spring collar 44. The manner in which this arrangement works is
identical to that shown in the FIGS. 11 and 12 embodiments.
Finally, FIGS. 15 and 16 show two arrangements in which the spring
system 11 consists, in each instance, of three springs. In FIG 15,
three coil springs 36, 37, and 50 are arranged coaxially above each
other with spring collars 44 and 44' interposed between them. The
springs are made of spring wire of unequal cross section, so that
they are all of different stiffness. When the spring system of FIG.
15 is compressed, the coils of the least stiff spring 36 first
contact each other, whereupon the spring 37 becomes effective. When
its turns contact each other, the spring 50 that has the greatest
stiffness becomes effective. This results in a spring system 11 of
progressive stiffness.
FIG. 16 also shows an arrangement with three springs. The stiffest
spring 50 bridges the whole of the spring stroke of the spring
system, whereas the two springs 36 and 37 that are not as stiff are
arranged concentrically and coaxially above one another and inside
the spring 50, with a spring collar 44 between them. It can be seen
that the spring system that results from these three springs of
unequal stiffness produces a progressive springing effect.
Numerous arrangements of springs that are arranged and combined
differently are possible within the context of the present
invention, in order to obtain the desired progressively effective
spring system. In each case, it is possible, in a simple way, to
satisfy all the demands made in practice by an appropriate
combination of the individual springs. In all the embodiments, it
is important that the spring system be of relatively slight
stiffness initially, when the induction regulator valve be open,
and that the stiffness of the spring system is relatively great
when the induction regulator valve is almost closed. In this way,
the sensitivity of induction regulation in the actual range of
regulation, with the induction regulator valve almost closed, is
reduced and this improves the precision of regulation as well as
overall regulation performance.
* * * * *